Time-of-flight (TOF) imaging techniques are used in many depth mapping systems (also referred to as 3D mapping or 3D imaging). In direct TOF techniques, a light source, such as a pulsed laser, directs pulses of optical radiation toward the scene that is to be mapped, and a high-speed detector senses the time of arrival of the radiation reflected from the scene. The depth value at each pixel in the depth map is derived from the difference between the emission time of the outgoing pulse and the arrival time of the reflected radiation from the corresponding point in the scene, which is referred to as the “time of flight” of the optical pulses.
Single-photon avalanche diodes (SPADs), also known as Geiger-mode avalanche photodiodes (GAPDs), are detectors capable of capturing individual photons with very high time-of-arrival resolution, of the order of a few tens of picoseconds. They may be fabricated in dedicated semiconductor processes or in standard CMOS technologies. Arrays of SPAD sensors, fabricated on a single chip, have been used experimentally in 3D imaging cameras. Charbon et al. provide a useful review of SPAD technologies in “SPAD-Based Sensors,” published in TOF Range-Imaging Cameras (Springer-Verlag, 2013), which is incorporated herein by reference.
For depth mapping with fine distance resolution, very fine temporal resolution of the TOF is needed. For example, for a distance resolution of 1 mm, the arrival time of the light pulses at the detector must be discriminated with a resolution of 6.6 ps. SPAD devices at room temperature, however, generally cannot reach this low level of time uncertainty. To overcome this problem, averaging and multi-measurement techniques have been proposed, such as time-correlated single-photon counting (TCSPC). In this technique, each measurement cycle begins with a START or synchronization signal, and concludes with a STOP signal provided by the SPAD upon arrival of the first photon in the cycle (assuming a photon arrives before the next cycle starts). A histogram of arrival times is typically built up over many cycles of this sort, and is then processed to locate the statistical peak.